Addition Polymerisation

Definition of Addition Polymerisation

Addition polymerisation is a type of reaction where monomers containing carbon 6carbon double bonds (C=C) join together to form a polymer chain. In this process, the double bonds break and the monomers link up without the loss of any small molecules. This is different from condensation polymerisation, where small molecules like water are released.

Monomers and Polymers

The monomers involved in addition polymerisation are usually alkenes, which have at least one C=C double bond. During polymerisation, these monomers open up their double bonds and join together to form long chains called polymers. The repeating unit in the polymer is derived from the original monomer.

For example, ethene (C₂H₄) is an alkene monomer. When many ethene molecules join together, they form poly(ethene), commonly known as polythene or polyethylene.

The repeating unit in poly(ethene) is 6CH₂ 6CH₂ 6, which comes from the ethene monomer after the double bond breaks and the monomers link up.

For instance, the polymerisation of ethene can be represented as:

Polymerisation Process

The key step in addition polymerisation is the breaking of the C=C double bond in the alkene monomer. This double bond consists of one sigma ( 3 4) bond and one pi ( 1 2) bond. The 1 2 bond is weaker and breaks during the reaction, allowing the monomers to join by forming new single (sigma) bonds.

As the double bond breaks, each monomer forms single bonds with neighbouring monomers, creating a long chain. This process repeats many times, causing the polymer chain to grow.

The general mechanism involves the following:

• Breaking the C=C double bond in each monomer
• Forming single bonds between carbon atoms of adjacent monomers
• Continuing the chain growth until many monomers are linked

For example, during the polymerisation of propene (C₃H₆), the double bond breaks and the monomers join to form poly(propene), with repeating units derived from the original monomer.

Example: The polymerisation of ethene involves breaking the C=C double bond in each ethene molecule, then linking the molecules together to form a long chain of poly(ethene).

Uses and Properties of Addition Polymers

Addition polymers are widely used as plastics in everyday life. Common examples include:

• Poly(ethene) (polythene) 6 used in plastic bags, bottles, and containers
• Poly(propene) (polypropylene) 6 used in packaging, ropes, and carpets
• Poly(chloroethene) (PVC) 6 used in pipes, window frames, and clothing

These polymers are generally inert and unreactive due to their strong carbon 6carbon single bonds and lack of reactive functional groups. This makes them durable but also means they do not biodegrade easily, leading to environmental concerns.